1. Field of the Invention
The present invention relates to an optical recording medium and to an optical recording and reproducing apparatus recording and reproducing information to and from the optical recording medium.
2. Description of the Background Art
A DVD (Digital Versatile Disc) has come to be practically used as an optical disk apparatus recording information with high density on an optical disk, using optical means. Further, an optical disk apparatus for high-density recording and reproduction by reducing the diameter of a laser spot converged on a recording surface of the disk by using an objective lens having high numerical aperture, as an objective lens for the DVD, has been proposed.
Generally, the diameter of the laser spot converged by an optical pickup is given by
xcex/NAxe2x80x83xe2x80x83(1)
where xcex represents wavelength of the laser beam, and NA represents numerical aperture of the objective lens of the optical pickup.
As represented by the expression (1), by enlarging the numerical aperture (NA) of the objective lens, the laser beam is converged to attain a small diameter of the laser spot which is incident on the optical disk, whereby the recording pit on the optical disk is made smaller, enabling high-density recording and reproduction.
It is difficult, however, to manufacture a single aspheric lens used as the objective lens having the numerical aperture of not smaller than 0.6. Further, in order to attain the spherical aberration of the laser beam within a tolerable range, it becomes necessary to make thinner the light transmitting layer of the optical disk as the numerical aperture is made larger.
An objective lens system having a large numerical aperture can be implemented by such a lens structure having two or more lenses as used in a microscope, for example. Considering the size and weight of the objective lens system, it is necessary to reduce the size and weight of such a lens structure when it is to be used in the optical pickup.
An objective lens system including two lens units of two lenses structure having a large numerical aperture includes an aberration adjusting mechanism for adjusting spherical aberration, in which one of the two lenses is movable to change the space between the lenses to adjust the spherical aberration. Thus, the spherical aberration experienced in the light transmitting layer is adjusted when a disk of which light transmitting layer has a thickness other than a prescribed thickness. Particularly, it is difficult to reduce spherical aberration of two or more light transmitting layers without the aberration adjusting mechanism, in a multi-layered disk having two or more light transmitting layers and two or more recording layers for recording information.
Actually, thicknesses of light transmitting layers and refractive indexes of optical disks are various and many dependent on the material or method of manufacturing the optical disks. Further, there are optical disks having light transmitting layer of uneven thicknesses generated during manufacturing. Therefore, the thickness of the light transmitting layer and the refractive index differ disk by disk, and therefore it is necessary to perform focus adjustment and spherical aberration adjustment every time an optical disk is changed.
Japanese Patent Laying Open No. 9-251645, for example, proposes a method of spherical aberration adjustment in a system having two lens units of two lenses, in which a lens mounted on the aberration adjusting mechanism is moved forward/backward from an initial value in a prescribed period, and the distance between the two lens units is so adjusted as to attain a position where a reproduced RF signal comes to have the maximum amplitude, that is, the lens position is adjusted to such a point where spherical aberration is small.
In the above described method, however, the initial value of the lens space of the aberration adjusting mechanism determining the distance between the two units at the time of aberration adjustment is calculated from the thickness of the light transmitting layer of the optical disk and the design value of the refractive index. Actually, it is necessary to change the initial value from disk to disk, because there is the difference in thickness of the light transmitting layers, unevenness of the thickness of light transmitting layers and the difference in refractive index of the optical disks.
Further, it is impossible to correctly record and reproduce information to and from a disk of which light transmitting layer has uneven thickness along the radial direction of the disk, as the thickness of the light transmitting layer changes after spherical aberration adjustment, resulting in increased spherical aberration.
For an optical disk having no record on it, or for a disk of which reflectance changes in one track, the position where the reproduced RF signal assumes the maximum value does not always correspond to the distance between the two lens units where the spherical aberration is small, and hence aberration adjustment is not possible.
The present invention was made in view of the foregoing, and its object is to enable correct recording or reproduction of information when two lens units are used.
The above described object of the present invention can be attained by an optical recording medium in accordance with one aspect of the present invention having at least one set of a light transmitting layer and a recording layer, on which information of thickness and information of refractive index of the light transmitting layer are recorded.
Preferably, an area on which the thickness information and refractive index information of the light transmitting layer are recorded is different from a general information recording area recording general information such as sounds and video images.
Further, preferably, the thickness information of the light transmitting layer is at least one of thickness value of the light transmitting layer determined before manufacturing, thickness value of the light transmitting layer obtained after manufacturing, and unevenness of the light transmitting layer obtained after manufacturing.
Preferably, the area on which the thickness information and the refractive index information of the light transmitting layer are recorded includes a first area on which the thickness value of the light transmitting layer and the refractive index determined before manufacturing are recorded, and a second recording area on which the thickness value and the unevenness of the thickness of the light transmitting layer obtained after manufacturing are recorded.
Further, preferably, the first recording area is a recording area pre-formatted at the time of manufacturing.
Preferably, the second recording area is rewritable after manufacturing.
Further, when there are a plurality of sets of light transmitting layers and recording layers, the thickness information and the refractive index information of the light transmitting layers are preferably recorded on a recording layer of any one of the plural sets.
According to another aspect, the present invention provides an optical recording and reproducing apparatus recording and reproducing information by converging a light beam onto an optical recording medium through two lens units, wherein distance between the two lens units is adjusted based on thickness information of a light transmitting layer and refractive index information of the light transmitting layer recorded on the optical recording medium having at least one set of a light transmitting layer and a recording layer.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.